Why do most molecular recipes use weight?

I'm not sure which recipes you're talking about, but a lot of baking recipes give weights instead of volumes because measuring ingredients by weight is much more accurate than measuring by volume.

If you want to test this, get a few friends to bring their measuring cups over to your house, and each of you measure a cup of brown sugar with your own measuring cups. Weigh the results of each. See how imprecise that was?

The main reason that weight is used when measuring out ingredients for molecular cooking is for accuracy as mentioned by Jack. With most home cooking, it can be easy to adjust recipes and ingredients which means that what you cook may not always turn out exactly the same. But with molecular cooking which is practiced mostly by chefs it is important to be able to come up with a perfect dish each time.

Because of this there are some things that need to be considered. For one, some of the ingredients used can be very potent and exact measurements are needed to get the best results. Also, you have to keep in mind that products from different companies can have different outcomes. Because of these the ingredients need to be measured for accuracy.

Also, I mentioned earlier that many of the ingredients used can be very potent. With the strength of these, very small amounts are needed. The best way to get exact amounts is by using weight measurements since these can be weighed out down to the last gram, no more and no less than what is needed. With the strength of the ingredients even a small excess or deficiency can be enough to affect the outcome of a dish.